One scheme for measuring true airspeed involves pointing a laser beam in the direction of flight and measuring the frequency shift in its reflection from bits of airborne dust. (Similarly, the speed of a train can be deduced from the perceived pitch of its whistle if the whistle’s true pitch is known.) I found several papers about this, going back to the 1990s, and they acknowledge the difficulties — for one thing, airborne particulates are scarce at high altitude — but conclude (as the authors of scientific papers are liable to do, since they hope to obtain funding to continue their research) that the problems are not insurmountable. The technology seems rather delicate and complex, however, and no one discusses its possible cost. I didn’t see any mention of other schemes, such as having the airplane eject its own aerosols and watch them recede into the distance, or eject particles laterally and time them as they go by. There seem to be lots of optical possibilities, at least for getting research grants and writing papers.
Another, perhaps less elegant, idea for directly measuring airspeed might come from the world of meteorology. Wind speed can be measured with what is called a sonic anemometer. The speed of transmission of a sound is relative not to the emitter but to the surrounding air, and so the transit time over a fixed distance changes if the air is moving. Sonic anemometers are quite expensive compared with the spinning-cup kind, but the basic components — ultrasonic emitters and receivers and microcircuits to translate delay time into a meaningful number — are cheap enough.
If, for example, you mount the emitter and the receiver 10 feet apart, the transit time of the audio pulse, which at 40 kHz or so is inaudible to humans but probably very annoying to bats, is on the order of a hundredth of a second. If the system moves through the air at 200 knots, that changes by 30 percent. So there would be no difficulty resolving speed differences of a fraction of a knot.
One paper described a scheme for ultrasonic airspeed measurement, but it involved a rather bulky tubular object attached to the outside of the airplane. I was imagining embedded speakers and microphones that would operate in the open air, say between the vertical fin and the cabin roof of a twin — the beam of sound would have to stay, as much as possible, outside the slipstream and the boundary layer — or between the tip of the horizontal stabilizer and the wing. But maybe the vicinity of an airplane is too noisy, even at ultrasonic frequencies. I trust that some informed reader — or perhaps a whole raft of them — will tell me that this idea, like most patentable-sounding fantasies of mine, has no merit whatsoever.
Cost-benefit analyses are the final resting place of many a technically sweet invention. To give the devil his due, pitot tubes seldom fail, and so we are talking about installing thousands of backup systems that might never be used. Competing with the airborne laser interferometer and the ultrasonic range finder is a small steel tube containing a heating element powerful enough to fry an egg. Obviously, the heated tube wins; it’s simple, cheap, easy to understand and service, and hard to break. Designing it is just a matter of getting the right MTBF — mean time between failures, the criterion of reliability that is of such small comfort to those present when a failure’s time has come. Evidently Thales, the manufacturer of the A330’s heated pitots, hadn’t gotten that part quite right.
